Epigenetic and Environmental Regulation of Myeloid Cells during Inflammatory Demyelinating Disease

Abstract

Multiple sclerosis (MS) is an incurable inflammatory demyelinating disorder of the central nervous system (CNS). Individuals with MS are burdened by neurological deficits affecting their motor, visual, and sensory function. These symptoms are clinical manifestations of inflammation, axonal swelling and transection, myelin destruction, and neuronal death within lesions of the brain, spinal cord, and/or optic nerve. It is widely accepted that MS is an autoimmune disease driven by autoreactive CD4+ T cells that, following entry into the CNS, recruit and activate other cell types. Histopathological analyses of CNS tissue from individuals with MS and mice with the murine model experimental autoimmune encephalomyelitis (EAE) have revealed substantial myeloid cell infiltration within inflamed lesions. Recent studies suggests that myeloid cells in the CNS during EAE and MS have pleiotropic functions. Myeloid cells contribute to pathogenesis by producing cytotoxic factors and presenting antigen to encephalitogenic T cells. Conversely, they can mitigate damage through the phagocytosis of harmful debris and release of anti-inflammatory cytokines. Understanding the factors that regulate CNS myeloid cell responses could lead to innovative therapeutic strategies for MS treatment. The aim of my dissertation project was to explore the role of two candidate regulatory pathways during EAE. The epigenetic profile of myeloid cells influences their phenotype and function in various models of inflammatory disease. Recently, ten-eleven translocation 2 (TET2), an epigenetic modifier that can suppress pro-inflammatory functions in myeloid cells, was identified as a susceptibility locus for MS. TET2 uniquely oxidizes methylcytosine to 5’hydroxymethylcytosine (5hmC) and facilitates the active demethylation pathway. TET2 and 5hmC are reduced in MS patient PBMCs compared with healthy controls. The objective of our study was to investigate the hypothesis that TET2 protects against aberrant myeloid cell activation during EAE, and that reductions in TET2 activity are a critical step in the development of a pro-inflammatory myeloid cell response. We found that Tet2-deficient mice develop more severe EAE following active immunization with myelin peptide. However, the transfer of myelin-specific CD4+ T cells into Tet2-deficient recipients did not result in similarly abrogated disease. This suggests that TET2 does not regulate myeloid cells in a clinically significant manner, but instead may regulate CD4+ T cell function. Inflammation in EAE and MS is preferentially targeted to certain CNS regions for reasons which are unclear. Regional differences in the intrinsic regulation of resident myeloid cell populations may mediate susceptibility to lesion formation. We have found that Axl and Mer, two members of the TAM family of tyrosine kinases, are highly expressed on microglia in the hindbrain, compared to the spinal cord, both during homeostasis and at EAE onset. In conventional EAE, inflammation localizes to the spinal cord. Blockade of Axl and Mer signaling using a pan-TAM receptor inhibitor (LDC1267) favored the development of an atypical, brain-targeted form of EAE. TAM receptor inhibition did not alter leukocyte populations in the hindbrain during EAE. However, it promoted the migration of leukocytes deeper into the brainstem parenchyma. Our data implicate TAM receptors as region-specific inflammatory regulators during EAE. Collectively, these findings contribute to our understanding of the regulation of myeloid cells in neuroinflammation. The presented data support a role of TAM receptors, and disputes a role of TET2, in controlling myeloid cell responses during EAE. Our studies provide insight that supports the development of more efficacious therapeutic options for MS patients.